Ambient light sensors based on CMOS photodiodes are used in a number of light sensing applications. For example, many mobile devices such as mobile phones and PDAs utilize backlit displays that represent a significant fraction of the power drawn from the batteries. In the absence of an ambient light sensor, such devices must set the power level of the backlight source relatively high so that the screen will be visible in high ambient light environments such as out of doors in bright sunlight. This results in a significant reduction in the operation time on batteries. To reduce the power drain, and hence, increase the battery-powered operation time of the device, the intensity of the backlight is adjusted to provide the correct light level based on the ambient light intensity in which the device is operating by incorporating an ambient light sensor in the device. If the device is being utilized outside in daylight, the ambient light intensity is high, and hence, the backlight source must be increased in intensity so that the display is easily visible. However, if the device is being utilized indoors in a low ambient light environment, the backlight source can be reduced in intensity, and hence, the power drain on the batteries can be reduced compared to the high ambient light
In a typical light sensor, the current generated by the photodiode is amplified to provide a signal that is linearly related to the intensity of the light received by the photodiode. While the current generated by the photodiode is linearly related to the light intensity of the light received by the photodiode, the amplifiers introduce non-linearities that limit the usefulness of such light sensors for some applications.
Various amplifier designs have been utilized to convert the relatively small signal generated by the photodiode to a current or voltage signal having an amplitude that is sufficient to be utilized by conventional processing circuitry. These designs suffer from non-linearities at high gain values and from dark current problems at low light levels. CMOS photodiodes generate a significant current when not illuminated by light in the typical amplifier circuits used to interface the photodiodes to the external circuitry. This current is often referred to as “dark current”. At low light levels, the dark current can be a significant fraction of the total current generated by the photodiode, and hence, the resulting output signal has an offset. This leads to one type of non-linearity, since the output signal is no longer proportional to the light intensity.
To correct for the dark current, a separate identical photodiode that is covered by an opaque layer is sometimes included in the light sensor and connected to a separate amplifier. The output of the separate amplifier is then subtracted from the output of the amplifier used to process the current from the light receiving photodiode. While this arrangement, in principle, corrects for the dark current, it introduces additional problems. First, the dark current limits the useful dynamic range of the illuminated photodiode and the amplifier associated with that photodiode. The amplifier has a maximum linear range. If part of that linear range must be utilized to amplify the dark current, then the range remaining for the amplification of the current from the light is reduced. Second, the separate amplification of the dark current also amplifies the noise associated with the dark current. This amplified noise is incorporated into the final output signal obtained by subtracting the amplified dark current from the amplified “light current”.
In addition to the problems associated with the removal of the dark current, the amplifiers utilized in prior art light sensors become non-linear at high output currents well before the signals reach the maximum levels that can be supplied by the amplifiers and power supplies. As a result, the prior art systems have a significantly reduced linear range that limits their usefulness in some applications.